In a papermaking machine, a wet web of paper or the like from the forming section of the machine is typically carried through the nip of a shoe press of the above-described type, where the web is pressed between two layers of absorbent felt or the like for wicking moisture from the web. Such shoe presses can also be used for calendering the web downstream of the forming section.
Various shoe presses of the above-described type have been proposed. For example, U.S. Pat. No. 4,917,768, which is commonly owned with the present application, discloses a shoe press in which the press shoe is carried on the support by tubular sleeves rigidly affixed to and spaced apart on the support along the cross-machine direction, the sleeves being slidably received within cylindrical recesses in the press shoe to permit the press shoe to be moved toward and away from a counter roll for varying the nip pressure. The shoe press includes hydraulic jacks upstream and downstream of the sleeves for urging the press shoe toward the counter roll and for pivoting the shoe about a cross-machine axis so as to vary the nip pressure in the machine direction. The sleeves fit somewhat loosely in the recesses in the shoe and a resilient seal encircles each sleeve for sealing the interface between the sleeve and recess. Accordingly, the press shoe is capable of pivoting relative to the support for varying the nip pressure in the machine direction.
One of the difficulties encountered in shoe presses is thermal expansion of the shoe from frictional heating of the shoe by the belt that carries the paper web through the press, as well as from hot hydraulic fluid which is circulated through the shoe for various purposes. Thermal expansion of the shoe causes elongation in the cross-machine direction. In the shoe press disclosed in the '768 patent, such thermal expansion of the shoe causes the sleeves to be placed under bending stresses, which is undesirable. Moreover, although the shoe in the '768 patent is slidable on the pistons of the hydraulic jacks, the large normal forces exerted on the shoe by the pistons during operation of the shoe press result in substantial frictional forces on the pistons when the shoe expands through thermal action. Consequently, the pistons are placed in bending within the cylinders of the hydraulic jacks, and such bending can lead to malfunction of the jacks, particularly for the cylinders toward the outer ends of the shoe farthest from the centerline where thermal expansion results in relatively greater translation of the shoe relative to the support and pistons. Bending of the pistons is undesirable from the standpoint of wear on the pistons, cylinders, and seals, and can also interfere with proper functioning of the press. Additionally, thermal expansion of the shoe can cause leakage of hydraulic fluid when the seals are excessively deformed.
In part because of the problems noted above with respect to the bending of the sleeves, the assignee of the present application developed an alternative shoe press similar to that disclosed in the '768 patent but eliminating the sleeves. However, this shoe press still suffered from the problems of bending of the pistons of the hydraulic jacks as noted above. Additionally, because the shoe was freely supported on the pistons of the hydraulic jacks, the shoe was free to take a variety of positions relative to the support beam and counter roll. More particularly, the shoe could become slanted, wherein one end of the shoe was displaced towards the downstream end of the machine and the opposite end was displaced toward the upstream end of the machine. Because of frictional forces between the shoe and the pistons, once the shoe became slanted it was difficult for the shoe to readjust into a correct position.